A method for image formation by diffusion transfer using a silver salt, such as a silver halide, is well known. This method comprises, for example, treating an imagewise light-exposed silver halide emulsion layer with an alkaline aqueous solution containing a developing agent and a silver halide solvent to reduce the light-exposed silver halide grains to silver with the developing agent and to convert the unexposed silver halide grains to a transferable silver complex salt by the silver halide solvent, diffusion-transferring the silver complex salt to a layer containing a silver precipitant (i.e., image-receiving layer) superposed on the aforesaid emulsion layer through inhibition, and reducing the thus transferred silver complex salt to a silver image with the developing agent with the aid of a silver precipitant.
A film unit which can be used in carrying out the above-described diffusion transfer process generally comprises a light-sensitive element comprising a support having provided thereon a light-sensitive silver halide emulsion layer, an image-receiving element comprising a support having provided thereon an image-receiving layer containing a silver precipitant, and a processing element comprising a rupturable container containing an active aqueous alkaline solution containing a developing agent, a silver halide solvent and a thickener. The emulsion layer of the light-sensitive element is imagewise exposed to light. The thus exposed light-sensitive element and the image-receiving element are then superposed on each other in such a manner that the emulsion layer and the image-receiving layer face each other and, in the interim, the processing element is ruptured and the superposed elements are passed through a pair of rollers so that a viscous alkaline aqueous solution is spread therebetween. After a given time has elapsed, the image-receiving element is peeled away from the light-sensitive element to thereby obtain a print having formed a desired image on the image-receiving layer thereof.
In order to improve preservability of the thus formed image for an extended period of time, various proposals have hitherto been made. In particular, U.S. Pat. No. 4,168,166 discloses a method for inhibiting generation of unfavorable crystals or salted-out compositions during storage of color images obtained by additive color photography, which method comprises using a borate compound as a component of a processing solution.
It was found, however, that when such a borate compound is used in a large quantity, generation of crystals or salted-out compositions during storage is sometimes rather accelerated.